Grease pencil 2.8 stroke placement problem
I just finded a workaround.

Go to Layers of Grease Pencil, and under the Relations drop menu you have to Parent the Layer to the Camera.

To predict better the draw radius I set up the grease pencil to draw with 3D Cursor and in Side (Y-Z) or Front (it depends). Remember that you have to place the cursor closer to the camera so it overlaps any other model in the scene.